The overall goals presented in this proposal are: (1) to continue the isolation of Vibrio cholerae mutants that express non-toxic proteins which are serologically related to cholera toxin (CRM); (2) to characterize these mutants and their tox gene products; (3) to begin preliminary in vivo experiments in gnotobiotic mice to examine in vivo reversion rates and in dogs to examine the effectiveness of various V. cholerae mutants as candidates for the development of a live oral vaccine for cholera.